This invention generally relates to digital data transmission systems and, more specifically, to the transmission of control data between different locations, for example to control multifunction telephones that are adapted for use in in private telephone networks.
Private telephone networks are generally used in corporations, institutions, and other organizations that require a number of telephones and a number of telephone trunks. There are two types of private telephone networks, each of which represents one application of this invention; they are: key telephone system networks and private branch exchange networks. Key telephone systems are usually used in small office systems whereas the private branch exchange is used in larger systems. Each type, however, has certain common characteristics and generic elements. For example, each type includes equipment at a central location that will perform some switching function for acoustic (e.g., voice) signals. U.S. Patent Application Ser. No. 855,181 discloses a private branch exchange that operates under the control of digital data processing equipment. This digital data processing equipment includes a central processor unit and memory units. The memory units store control information and control programs for enabling the central processor unit to operate the private branch exchange, especially basic call processing functions that include call switching. However, the use of digital data processing equipment as a control mechanism also enables a number of other functions to be performed easily and inexpensively.
Some examples of such functions include conference, "camp-on", call forwarding, and automatic calling functions. The conference function allows a first party to talk simultaneously with a second party and to add and drop third parties from the conversation. If the "camp on" function is available and the first party is talking with a second party when an incoming call is received from a third person, the "camp on" function places the incoming call into a hold condition. When the first and second parties complete their telephone conversation, the first party receives a signal indicating that an incoming call is on hold. The conversation can then begin. If a party in an office temporarily moves to another office, the call forwarding function enables the private branch exchange to automatically transfer all calls to another extension that is designated by the party. If a party dials a telephone number and that line is busy, the automatic calling function allows the number to be redialed merely by pushing a single button.
A number of existing private telephone networks perform one or more of the foregoing and other functions. Generally, a telephone in such a network is known as a multifunction telephone and includes a number of control, or function, pushbuttons on the telephone. Common equipment, including data processing equipment, polls the telephones in sequence by sending a message to the telephone and then receiving a message indicating that a button has been activated. The data processing equipment processes other information in conjunction with received messages to identify a specific function. Control programs in the data processing equipment define each function. This provides a very flexible private telephone network as it is merely necessary to change or add a control program to change or add a function. Thus, a standard mechanical configuration can provide a very flexible set of functions which a customer can then individually select for his particular application and needs.
In both key telephone systems and private branch exchange networks uninterrupted cables interconnect the common equipment at the central location and individual telephones at remote sites. The cables that interconnect the common equipment and multifunction telephones include at least two separate signal paths. One signal path constitutes an acoustic, or voice, signal path and another signal path constitutes a data signal path for control signals. Each signal path includes a pair of conductors to ensure the reliability of transmissions over these paths. Reliability is also ensured by specifying a maximum length for the data signal path and, therefore, the cable. This maximum corresponds to a loop resistance of about 200 ohms. The actual maximum length depends upon wire size, but conventionally the maximum length is from 2,000 to 3,000 feet.
More specifically, some signal encoding and decoding is used to convey the data signals over the data signal path. In some networks, for example, digital data signals are converted to diphase or Manchester phase representations for transmission over the data signal path. As the encoded signals travel along the data signal path, they degrade. One particularly important degrading influence is amplitude attenuation, because the detection and decoding process is sensitive to signal amplitude. At some data signal path distance the data signals can no longer be reliably separated from noise. This distance also corresponds to an impedence of about 200 ohms. One apparent solution, that would greatly increase the maximum signal path length, is the installation of conventional modems. However, that solution has not been practicable for private telephone networks due to expense. Each line would require a conventional modem at the common equipment and another conventional modem at the telephone. A conventional modem includes a clock, a modulator for converting digital signals in serial form to analog signals and a demodulator for converting analog signals to digital signals in serial form. The modem also includes serial-parallel conversion means to couple the serial digital data at the modulator and demodulator to parallel signals for a digital interface at the respective ones of the common equipment or telephone. As is apparent, there are increased costs if two modems are to be incorporated in each line; and in a private telephone network, the overall increase can be prohibitive. Moreover, the overall size of the telephones would have to be increased to accomodate the modem. These alterations also would increase the cost of the telephone.
All these costs and other problems have made it impractical to use conventional modems in a private telephone network. As a result, the private telephone networks that include multifunction telephones accept the maximum signal path limitations. Thus, commercially available networks that have a multifunction telephone capability normally limit the signal paths to a range from 2,000 to 3,000 feet.
In the currently available telephone networks, the central processor unit at the common equipment exclusively controls all operations in the multifunction telephone. This eliminates much of the circuitry that would otherwise be incorporated in a multifunction telephone. Thus, the telephone itself can be manufactured at a low cost. However, the central processor unit must perform the additional operations. At some point, the central processor unit is no longer able to perform all the operations and still maintain adequate switching speeds. The least expensive solution, then, is to replace the central processor unit with one of greater capability and increased memory. However, this also increases the cost of the common equipment.
Another possible solution is to distribute functions to the telephone. However, this is a difficult task because the telephone is an "event-oriented" device. Typical events include removing or replacing the handset and dialing. Normally encountered procedures for controlling such a central processor unit coupled with the execution times for performing these procedures can lead to timing and synchronization problems. Obviously a central processor unit of sufficient speed and capability might be used successfully. However, sufficient speed and capability are achieved only with equipment that would increase the telephone costs to prohibitive levels.
Moreover, many of these private networks utilize common equipment that is especially adapted for multifunction telephones. Modification of a currently installed private telephone network for conventional telephones may be possible only at significant expense.